1. Field of the Invention
The present invention relates to orthopaedic reamers, and, more particularly, to chip breaker cutting apertures and surfaces for orthopaedic reamers.
2. Description of the Related Art
A hip joint prosthesis requires preparation of the acetabulum by milling a precision shape therein using an orthopaedic reamer. A typical orthopaedic reamer has a hollow hemispherical shell shape with apertures in the hemispherical shell. The hemispherical shell is attached to a driver with a shaft which can be inserted into a rotating tool such as a drill thereby providing rotation of the orthopaedic reamer, and hence, the motive force for the milling operation. The apex of the hemispherical shell typically is along the shaft longitudinal axis.
The apertures in the hemispherical shell have a dual purpose. Firstly, an edge of each aperture is formed outwardly, respective to the shell center, to form a cutting tooth or cutting surface. The forming of the aperture edge outwardly is referred to as a lipping operation. Secondly, the apertures allow milled tissue to collect in the shell interior, thereby providing a somewhat self-cleaning aspect to the milling operation. The apertures and cutting surfaces are located on the shell to provide approximately 180° cutting coverage during rotation of the acetabular reamer thereby allowing a uniform milling of the acetabulum.
The size, shape, quantity and location of apertures and corresponding cutting surfaces are interrelated. For example, smaller geometry apertures and corresponding cutting surfaces require a greater quantity in the hemispherical shell to provide approximately 180° cutting coverage during rotation of the acetabular reamer. Conversely, fewer relatively large size apertures and corresponding cutting surfaces are required to provide the same cutting coverage. The size and shape of the cutting apertures also influences the location of the apertures. For example, one known design of acetabular reamer has approximately 0.2 inch diameter round aperatures arranged in a spiral pattern in the reamer hemispherical shell.
When the reamer is used to prepare the acetabulum for hip joint prosthesis, variation in the size of the acetabulum for the human population requires a range of sizes of acetabular reamers, a specific size of the acetabular reamer being determined by the hip joint size of the person undergoing hip joint prosthesis. The different sizes of acetabular reamers are generally specified by different radii of curvature, or diameters, of the hemispherical shell. For a given size and shape for apertures and corresponding cutting surfaces, a larger reamer size requires a larger quantity, and possibly different locations.
Manufacturing cost for a reamer is incremented for additional apertures and corresponding cutting surfaces; therefore, larger and fewer cutting apertures save manufacturing cost. When the reamer is used to prepare the acetabulum for hip joint prosthesis, a rotary hand tool provides the motive force and is connected to a driver which is connected to a reamer. In use, larger cutting surfaces remove more material for a given rotation of the tool leading to vibration or chattering. This vibration or chattering can degrade cutting performance by making the rotary tool-driver-reamer combination harder for the operator (surgeon) to control. Smaller cutting surfaces remove smaller pieces of material, thereby improving the vibration characteristics of the rotary tool-driver-reamer combination, but require a larger quantity of cutting surfaces, thereby increasing the manufacturing cost of the reamer. In other words, there is a trade-off between the manufacturing efficiency of fewer, larger cutting apertures and the cutting and vibration performance of a greater number of, and smaller, cutting apertures.
What is needed in the art are reamer apertures and corresponding cutting surfaces with the manufacturing efficiency of larger cutting apertures and the cutting, chip breaking and vibration performance of smaller cutting apertures.